Hereinafter, a general modulation technology for use in data communication will be described in detail.
A term “modulation” represents that signal information is modified into another suitable for channel characteristics of a transmission medium. For example, intensity, displacement, frequency or phase of the signal information may be modified according to the channel characteristics of the transmission medium. If the signal information is modulated using a modulation method suitable for the transmission medium, the signal information can be transferred to a long distance.
A user is able to construct several frequency channels in a wide frequency band. The user modulates specific data, such that it can provide a subscriber with data over the several frequency channels. Also, as a frequency gradually increases in case of modulating data, an antenna length is gradually shortened in proportion to the increasing frequency. In addition, in case of modulating data, the user satisfies design requirements such as a filtering or amplification, and at the same time modulates data. That is, the term “modulation” represents that a signal having data is modulated into a waveform suitable for a transmission channel.
A phase shift keying (PSK) modulation represents a modulation scheme in which a carrier phase is changed to another according to information of a digital signal. The PSK waveform has a certain amplitude or envelope, such that it is less affected by a variation of a level and has a high immunity from a symbol error. A modulated wave is a double sideband signal, such that it has both timing information and frequency information. Therefore, a modulation/demodulation circuit can be configured in the form of a relatively-simplified structure.
A binary PSK (BPSK) modulation matches transmission signals of two values (e.g., 0 or 1) to be transferred with an in-phase and an out-of-phase, respectively, such that it transmits the matched resultant signals. A quadrature phase shift keying (QPSK) modulation collects two bits of ‘0’ and ‘1’ corresponding to digital signals of the two values, matches the collected bits with 4 phases of a carrier, and transmits the matched result.